© The Board of Management and Trustees of the British Journal of Anaesthesia 2004
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Density of spinal anaesthetic solutions
Editor—I read with interest the article by Dr McLeod on the density of spinal anaesthetic solutions.1 Dr McLeod has done excellent work in measuring the densities of the solutions. We know that density has a major role in the spread of spinal anaesthesia.I was wondering if there is a printing error in Table 2? According to the table, the density of bupivacaine 7.5 mg ml–1 is less than that of bupivacaine 5 mg ml–1. Likewise, the density of ropivacaine 10 mg ml–1 at 37°C is less than that of ropivacaine 7.5 mg ml–1. On the other hand, in the text there is mention that ‘increasing the concentration of bupivacaine ... significantly increased the density’, which is what one would expect. Which numbers are correct? This is important because Dr McLeod has put major effort into his work and these valuable numbers could be referred to in future studies.
In addition, on page 548, solutions, third line from the bottom: should the dilution of ropivacaine be 10 mg ml–1 not 1 mg ml–1?
Helsinki, Finland
Editor—May I thank Dr Pitkänen for taking an interest in my paper and highlighting these points.
With regard to plain solutions, there was a significant increase in the density of bupivacaine 5 mg ml–1 and 7.5 mg ml–1 compared with bupivacaine 2.5 mg ml–1 using one-way ANOVA. There was no difference in density between the 5 mg ml–1 and 7.5 mg ml–1 solutions of bupivacaine. There was a significant decrease in density between ropivacaine 2 mg ml–1 and ropivacaine 10 mg ml–1 using one-way ANOVA. In contrast, the density of levobupivacaine significantly increased from concentrations of 2.5 mg ml–1 to 7.5 mg ml–1. The reason for the discrepancy is to be found in Table 5 of my paper.1
The contribution of electrolyte composition to overall density varies between drugs. With bupivacaine and ropivacaine, there is a reduction in the concentration of sodium as the concentration of local anaesthetic is increased. In contrast, with levobupivacaine, no corresponding reduction in sodium concentration occurs and, hence, osmolarity and density both increase. I hope this clarifies the point.
May I add that all densities should be expressed in g ml–1 and not mg ml–1 as I have written. I have asked for an erratum notice to be published to acknowledge my mistake.
Dundee, UK
References
1 McLeod GA. Density of spinal anaesthetic solutions of bupivacaine, levobupivacaine, and ropivacaine with and without dextrose. Br J Anaesth 2004; 92: 547–51
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